1. Field of the Invention
The present invention relates to a photomultiplier that enables a cascade-multiplication of secondary electrons by emitting sequentially the secondary electrons through a plurality of stages in response to incidence of photoelectrons.
2. Related Background Art
In recent years, developments of TOF-PET (Time-of-Flight-PET) are earnestly proceeding as a PET (Positron-Emission Tomography) apparatus for the next generation in the field of nuclear medicine. In particular, in the TOF-PET apparatus, when two gamma rays emitted from a radioactive isotope administered in a body are simultaneously measured at two detectors in directions opposite to each other, a time difference in signals outputted from the two detectors can be determined, which enables to determine a disappeared position of positrons as a difference in flight or transit time; thus, it becomes possible to obtain a vivid image of the PET. A photomultiplier with a large capacity having an excellent high-speed response is employed for the detectors.
For example, a photomultiplier shown in JP-A-5-114384 is known as the aforementioned one. In the conventional photomultiplier has a construction such that a focusing electrode and an accelerating electrode are arranged in this turn from a cathode toward a first-stage dynode. In this case, the focusing electrode is the one correcting an orbit of each photoelectron emitted from the cathode such that the photoelectrons may be focused on the first-stage dynode. In addition, the accelerating electrode is the one accelerating the photoelectrons emitted from the cathode to the first-stage dynode, and has a function to reduce variations in transit time from the cathode to the first-stage dynode caused by the emission area of the photoelectrons of the cathode.
A high-speed response can be achieved by the configuration arranging the focusing electrode and accelerating electrode between the cathode and the first-stage dynode, as mentioned above.